How to measure body weight on a lever scale. Measuring body weight on lever scales. Small weights should only be picked up with tweezers

Goal of the work: learn to use lever scales and measure mass with them.

Equipment: scales, weights, a piece of plasticine, a beaker, a flask with water, a small bottle with a stopper (Fig. 120).

Rice. 120

Test yourself

Answer the questions.

1. What does body weight depend on?
2. What is the mass of a piece of plasticine glued together from three pieces weighing m 1 = 200 mg, m 2 = 40 g, m 3 = 0.60 kg?

Weighing Rules

Balance the scales by adding pieces of paper to the lighter cup.

Place the body to be weighed on the left pan of the balanced scale.

Holding the scales with your hand, carefully place a weight on the right side of the scale, the mass of which, in your opinion, is equal to the weight of your body. If it turns out that the weight of the weight is greater than the weight of the body, then remove this weight and place a weight of lesser mass. Balance the scales by adding small weights. Remove small weights (from 500 to 10 mg) from the case using tweezers.

Calculate the total mass of all weights. It is equal to the mass of the body being weighed.

Do not place wet, dirty, or hot bodies on the scales, pour powders without using a liner, or pour liquids.

Once you have finished weighing, place the weights in their slots in the box.

Progress:

Control questions

1. How to measure the mass of one grain of rice if the mass of any of the weights in the set of weights is greater than the mass of one grain?
2. When weighing the body, on the right pan of the balanced scales there were two weights of 200 g, weights of 50 g, 10 g and two weights of 100 mg. What is the body mass in grams (g)? In kilograms (kg)?
3. Can it be considered fair to say that a body with a larger volume has more mass than a body with a smaller volume? Justify your answer.
4. Having only a scale with a weight and a plate with side a = 3 cm cut from a uniform sheet of cardboard, determine the area of ​​an irregularly shaped plate (Fig. 121) cut from the same sheet of cardboard.

Rice. 121

Laboratory scales vary in purpose, design, weighing range and other characteristics.

Weighting methods are divided into two fundamentally different groups - the method of comparison with a measure and the method of direct assessment. According to the method of comparison with a measure, the mass of the load is taken equal to the mass of the weights compared with it (simple weighing) or is calculated as the sum of the mass values ​​of the weights and the readings of the scales (precise weighing). The direct assessment method consists in determining the mass of the load using the reading device of the scales without the use of weights.

Most modern laboratory scales use a differential weighing method, in which most of the measured body mass (over 99%) is balanced by weights or a counterweight (zero method), and the remaining small difference between the mass of the body being weighed and the mass of the weights is measured by the angle of deflection of the rocker arm from the initial position equilibrium (direct method) using reference scales.

Laboratory scales are characterized by a number of parameters. The main ones are the following.

1. Maximum permissible load, within the range of which the error of indications is within the established limits. Do not exceed the maximum permissible load for which this scale model is designed. Too much load can cause permanent deformation in the rocker arm, which will damage the balance.

2. The permissible error of readings is the maximum difference between the actual value of the mass of the load being weighed and the readings of the scales. The error value characterizes the correctness of the weighing results under standard conditions and cannot be less than the possible errors of the weights used during weighing and certification of scales.

3. Permissible variation (inconstancy) of readings - the maximum permissible difference in readings of scales when repeatedly weighing the same load under standard conditions using the same weights. The variation value characterizes the reproducibility of the weighing result and, to a large extent, the weighing accuracy.

4. Sensitivity is the limiting ratio of the increment in the deviation of the scale pointer to the increment in the measured value. Sensitivity is determined by the number of scale divisions by which the scale arrow deflects when a load weighing 1 mg is placed on one of the scales. Express sensitivity in scale divisions per milligram or its reciprocal value.

As the load on the cups increases, the sensitivity of the scales decreases, i.e., the greater the mass of the object being weighed, the weaker the scales react to changes in mass.

5. Division price - the division value of the reading devices. Often the division price is consistent with the permissible error value or the variation in the scale readings.

6. Performance - the possible performance of work on the scales, i.e. the possible number of weighings per unit of time.

Classification of scales

According to their purpose, laboratory scales are divided into technical (general laboratory), analytical and special, and weights - into general use and special weights.

The largest weighing limits of technical scales are in the range of 20 g - 50 kg. The most common scales have a load of 0.2-5 kg, with a division price of 0.05-0.1 g.

Analytical balances are used for macro- and microchemical analyzes when weighing the highest and highest accuracy. Depending on the maximum permissible load and division price, analytical balances are divided into the following groups:

Special scales are used to determine quantities depending on mass (weight moisture meters, scales for measuring magnetic susceptibility, etc.).

Analytical group scales belong to accuracy classes 1 and 2, technical scales - to classes 3 and 4. The average reduced weighing error for class 1 scales is 0.0001%; 2 classes - 0.0005%; 3 classes - 0.001%; 4 classes - 0.01%.

General laboratory weights are divided into four classes. Weights of classes 1 and 2 are intended mainly for analytical balances, classes 3 and 4 - for technical ones.

Based on the nature of movement of the moving system, scales are divided into leverless and lever scales. In non-lever scales, the moving system moves back and forth vertically, so weights cannot be used to balance the load being weighed. When using leverless scales, only the method of directly assessing the weighing results is suitable.

Lever scales are characterized by rotation of the moving system around a fixed or conditionally fixed axis. They can be weighted (with overhead or built-in weights) and weightless. Scales with built-in weights are more productive and convenient, but they make it difficult to control the actual mass values ​​of the weights.

Lever scales vary in the type of weigh arm supports and hangers. The most common rigid support is a cushion on which a prism rolls with its sharp edge. Scales with such supports are called prismatic. Prismatic scales are divided into equal-arm, double-prism (single-cup) and quadrant.

Equal-arm scales are basically a lever of the first kind, in which the distances from the application of forces to the fulcrum are equal (Fig. 71). If you place a load with mass M1 on the left pan of the scale, then to return the arrow P to its original position, you will need to place a certain amount of weights (with a known mass) on the right pan. When equilibrium is established, the moments of force acting on the left and right parts of the rocker at the points on which the cups rest, at a distance l1 and l2 from these points to the support point, will be equal: F1l1 = F2l2.

Since l1 = l2, then, therefore, when equilibrium is achieved, F1 = F2. The emergence of forces F1 and F2 is associated with the attraction of bodies on the scales by the Earth. Force F1 determines the attraction of a body with mass M1 to the Earth, i.e. its weight. The unit of weight is newton (N). A Newton is equal to the force that imparts an acceleration of 1 m/s2 to a body weighing 1 kg in the direction of the force. The weight of a body G is related to its mass by the following relations: G = Mg, where M is the mass of the body, and g is the acceleration of gravity. The unit of mass is kilogram (kg).

From the above it follows that scales are devices for determining mass, not weight.

Equal-arm rocker scales are shown in Fig. 72. The equilibrium position of unloaded scales is called the zero point, loaded ones - the equilibrium point.

To protect the ribs of the rocker prisms from damage and rapid wear, all moving parts of the scale can be raised and the ribs of the prisms separated from the plates with which they come into contact. The device used to lift the rocker and earrings is called a locker (isolator). When scales are not in use and when objects and weights to be weighed are placed on cups, the scales must be locked.

Until recently, V-shaped depressions of the rater scale were applied to the rocker arm of equal-arm analytical balances at equal distances from each other (Fig. 73), into which a rater weight of 10 or 5 mg was installed using a special device. By moving the rater along the rocker arm, it was possible to determine the mass with an accuracy of tenths of a milligram.

Modern prismatic scales have vibration dampers for the scale pointer - dampers. In damper scales, the zero point and the equilibrium point are taken to be the scale division opposite which the pointer stops. For scales that do not have dampers, these points are determined by the swing method. This method is based on measuring 3-5 consecutive needle deflections. The first 2-3 fluctuations after turning on the scales are not taken into account, and the next 5 deviations of the arrow in one direction or the other are recorded with an accuracy of tenths on the scale. The zero point is calculated, for example, as follows.

Deviations to the left: -3.4 and -2.8; the average is -3.1.
Deviation to the right: +4.0, +3.5 and 3.0, average +3.5.
Let's find the sum of deviations: +3.5 + (-3.1) = 0.4.
Let's find the zero point: +0.4: 2 = +0.2.

The accuracy of damper scales is of the same order as the accuracy of conventional scales.

Double-prism (single-cup) scales are shown in Fig. 74. In the initial position, all built-in weights are loaded on the suspension and the lever is balanced by a counterweight. Having placed a load on the load-receiving cup using a special weighting mechanism, such a number of built-in weights are removed from the rack so that their total mass approximately corresponds to the weight of the load. The difference between the mass of the load and the mass of the removed weights is determined by the readings of the reading device. Double prism single pan balances are used primarily as analytical balances. The advantages of this design of scales are that work is always carried out with a constant load on the rocker arm, and in this case both the sensitivity of the scales and the weighing accuracy are constant.

Quadrant scales or scales with an upper position of the load-receiving bowl (Fig. 75) are a type of double-prism scale.

Lever scales with supports on elastically deformable elements are produced for weighing not large masses. These include torsion bar scales and spring lever ultra microbalances.

General laboratory equal-arm scales

General laboratory equal-arm scales - technical scales mainly of 3rd and 4th accuracy classes - are used for weighing relatively large masses. They can be enclosed in a glass display case and equipped with a weight mechanism with built-in weights, or they can be hung on a stand mounted on a stand without a weight mechanism. The simplest type of equal-arm scale with two cups is a manual or pharmacy scale.

Technochemical scales of the VLT-200g (T-200) and VLT-1kg (T-1000) types are shown in Fig. 76. When weighing, by turning the lock handle, the scales are brought into working position. The permissible error for VLT-200g scales is ±60 mg, for VLT-1kg ±200 mg.

More advanced technochemical scales of the VLR-1kg type consist of an equal-arm rocker with a pointer, a column with a support pad, an insulating device and two load-receiving cups suspended on the end prisms of the rocker. The scales are equipped with an oil vibration damper of the rocker arm and a device for mechanical weighting of built-in weights (from 10 to 990 mg).

Before weighing, make sure the scale is level and installed correctly. If necessary, the scales can be installed strictly horizontally using screw feet. Then you need to check the deflection of the arrow and ensure that it is completely aligned with the control stroke of the scale dial.

IN last years equal-arm technical two-cup scales of the VLT type were significantly modernized and serial production of a number of new models of scales of the VLR type was carried out, class 2 accuracy (with an error of ±10 mg), with a load capacity of 1, 10, 20 and 50 kg, with a scale division of 10 mg.

The VLR scales are placed in a glass case with doors on two sides. At the upper end of the column there is a cushion on which the middle prism of the rocker rests with its edge. An oil damper is installed at the base of the column. At the ends of the rocker, prisms are fixed in special saddles, onto which earrings with load-receiving cups are hung. Ring weights (from 100 to 900 mg and from 10 to 90 mg) connected to the major and minor limbus are hung and removed from the bar attached to the right earring using a weight mechanism.

There is an arrow in the middle of the rocker, and at the bottom of the column there is a scale on which the balance of the scales is checked. An insulating device (lock) is mounted under the base of the scales. The lock must be opened and closed carefully, by smoothly rotating the handwheel at the moment when the scale arrow passes past the zero mark of the scale.

General laboratory quadrant scales

In recent years, quadrant scales have become widespread due to their speed of operation. These are two-prism scales with an upper cup position. Magnetic vibration damper. There is an optical device and a screen on which the weighing results are read. The application and removal of overhead weights is carried out using a handle located on the metal body of the scale. The scales are connected to the alternating current network through a built-in transformer mounted under the scale display case.

Quadrant scales are intended for determining the mass of various substances and materials during laboratory technical analyzes and preparative work.

The operating principle of the scales is based on balancing the moment of forces created by the measured mass, quadrant deflection and built-in weights.

Currently, six modifications of laboratory quadrant scales of class 4 VLKT and VLK are produced with weighing limits from 160 to 10,000 g.

VLKT scales (Fig. 77) have a tare compensation mechanism, which allows you to increase weighing performance and is designed to set the scale to zero after placing the tare on the scale pan.

The value of the measured body weight on the scale is found by summing the readings on the optical scale and on the counter. The number of hundreds or thousands of grams is counted by a counter, in the window of which the numbers 0, 1, 2, 3 and 4 appear, depending on the mass of the weights removed from the suspension.

Weighing on technical scales

The scales are installed on strong, stable tables in the laboratory working area strictly vertically and plumb. Before weighing, check whether the scales are installed correctly, then lower the rocker arm with a lock and observe the oscillations of the needle on the lower scale. If the arrow deviates from zero by the same number of divisions to the right and left, the scale can be used. Otherwise, the balance of the scales is achieved using the balancing nuts of the rocker arm.

The mass to be weighed is placed on the left platform of the scale, the weights of the gram set are placed on the right, and the weights of the milligram set are hung by the weight mechanism.

It is better to determine the mass of a substance using the double weighing method, which consists of the following: place the object being weighed on the left pan of the scale, and weights on the right pan until the scale needle reaches the zero mark of the scale. After this, the object to be weighed is transferred to the right cup, and the weights to the left. If one of the cups outweighs the other, then by adding or removing weights, the equilibrium point is again established. The actual mass of the object being weighed is equal to the arithmetic average of the results of these two weighings. At the end of weighing, the object being weighed is removed from the scales, the weights and weights are removed, placing them in the prescribed order in the case.

Analytical balances

To an even greater extent than technical and technochemical balances, the balances of the analytical group have undergone modernization in recent years. At the same time, many chemical laboratories still successfully use equal-arm rater scales without dampers - periodic swing scales not equipped with built-in weights. The peculiarity of working on periodic swing scales is to determine their zero point. The rocker arm, freed from the arrester, begins to perform gradually damped oscillations. The zero point and the equilibrium point are determined by the method of repeated deflections of the rocker arrow. Before determining the zero point, the rater must be removed from the rocker arm if the zero is in the center of the rocker arm, or set to zero if the zero is at the left end of the rocker arm.

To determine the sensitivity of the balance, set the equilibrium point under various loads. To do this, after establishing the zero point, place the rater on the rocker arm (with locked scales) so that it shows 1 mg, lower the arrester and determine the equilibrium point.

For example, if the zero point of the scale is +0.2 divisions, and the equilibrium point with a load on the right cup of 1 mg is +3.8 divisions, then the sensitivity of the scale is found by placing 5, 10, 20, 30, 40 on both cups in succession , 50 and 100 g. The results obtained are plotted on a graph.

When using rater damper scales, mass determination using built-in or overhead weights is carried out only up to 10 or 5 mg (i.e., up to the weight of the rater). Further balancing is carried out using a rater, which is set only to the nearest whole milligram to equilibrium. If weighing accuracy greater than 0.1 mg is not required, tenths of a milligram are found by moving the rater along the rocker. If more accurate weighing is necessary, the rater is installed as in the previous case, and tenths and hundredths of a milligram are found by the difference between the zero point and the found equilibrium point based on the previously determined sensitivity of the scales for a given load.

Let, for example, the zero point of the scales be +0.5; the balance point of the scales with a load of 14.3300 g on the right pan and the rater on the 3 mg mark is +2.0; The sensitivity of the scale with a load of 14.5 g is 4 divisions per 1 mg. Obviously, the object being weighed is not completely balanced. If the rater is moved to the 4 mg division, then the equilibrium point will move 4 divisions to the left, i.e. it will be equal to -2.0. In order for the equilibrium point to coincide with the zero point (+0.5), the rater must be moved by (2.0 - 0.5)/4.0 = 0.38 divisions, i.e. by 0.38 mg. Therefore, the mass of the object being weighed will be equal to 14.3300 g (on the scale) + 0.00038 g (reading by the rater) - 14.33038 g.

Many laboratories use two-cup equal-arm laboratory analytical balances VLA-200 g-M (AD-200) with the following basic characteristics: maximum permissible load 200 g; optical scale measurement range ±10 mg; the error due to the unequal arms of the rocker is no more than 2 mg. The weights are controlled using dials. When the small dial rotates, tens of milligrams are added or removed, and when the large dial rotates, hundreds of milligrams are added. The dials rotate independently of each other. The scales are turned on and off using a handle placed on the locking roller placed on the front wall of the base.

Currently, the industry produces mainly two-cup equal-arm scales of the VLR type, for example, scales of the accuracy class VLR-200g and VLR-20g. The VLR-20g scales, replacing the VLM-20g-M semi-microanalytical balances, are distinguished by their high sensitivity and smaller overall dimensions. On the basis of the VLR-200g scales with an electronic attachment, the VLE-200g electronic scales are produced.

Technical data of the VLR-200g scales (Fig. 78) and the VLR-20g scales are given below:

When using scales of the VLR-200g type, first of all, turn on the illuminator in the network, after which, without opening the doors of the scale cabinet, carefully turn the locking disk until it stops. An electric light that lights up automatically illuminates an enlarged image of a microscale attached to the scale needle on the weightograph screen. If the scales are not loaded, the zero of the scale must exactly coincide with the vertical line on the screen (mark). Otherwise, coincidence is achieved by rotating the adjusting screw located outside on the lower board of the scales above the locking disk. Then the load is placed on the left pan of the scales, and on the right - gram weights from the set of weights for the scales; in this case, find the mass of the number of whole grams. Close the cabinet door; turning the small dial with tenths of a gram, combine the fixed pointer with the various numbers on the disk. Each time you turn the dial, you must first lock the scales. Having established the number of tenths of a gram, find hundredths of a gram using a large dial. Next, the locking disc is turned until it stops and, after the arrow of the rocker arm stops oscillating, the position of the vertical line is measured on the scale on the screen. Large divisions of this scale, corresponding to milligrams, are indicated by numbers with a “+” or “-” sign. The plus shows that the value of the reading made must be added to the mass of the weights placed on the scales, and the minus must be subtracted.

After the weighing is completed, the result is recorded, the weighed object and weights are removed from the scales. To release the rocker from the built-in weights, rotate the disc handles until the fixed pointer aligns with the zero division of both discs.

In addition to equal-arm analytical balances of the VLR type, the industry produces class 2 single-arm balances of the VLDP-100g type (Fig. 79). The principle of weighing on double-prism scales is based on balancing the moment created by the load and the moment obtained when removing the built-in weights from the suspension. The balance beam is an unequal lever; a saddle with a load-receiving prism is attached to the short arm, and a reading scale to the long arm. An earring rests like a cushion on the load-receiving prism of the rocker, to which a bar is rigidly attached for applying built-in weights. A weight mechanism is used to remove and apply built-in weights. Simultaneously with the removal of the weights, the value of their mass (in g) is displayed in the three left windows of the screen. For precise weighing, the rocker arm is calmed using an air damper; with preliminary – oil. The handle for putting the scales into working position is located on the left side of the scales. Preliminary weighing is carried out by turning the handle away from the operator, and precise weighing is carried out by the operator. The zero position of the scale during preliminary weighing is adjusted with the handle located with right side scales, above; for precise weighing - with the handle at the bottom. The pre-weighing mechanism is designed to determine the mass of the built-in weights. To take a reading on the scale, there is a reference mark on the screen in the form of two parallel lines.

The weighing result is determined by the sum of the readings of the reading scale, the counters of the weight mechanism and the dividing device. Weighing range from 0 to 100 mg. The price of the smallest scale division is 0.05 mg. Weighing error ±0.065 mg.

Installation of analytical balances

Installation of analytical balances begins with the selection of premises and organization of the chemist's workplace. The room for installing scales of classes 1 and 2 should consist of a weighing room and a preparation room. One of the conditions imposed on the weighing room is its complete isolation from adjacent laboratory premises.

For the weight room, choose a bright, dry room. It is desirable that it be located on the ground floor, with windows facing north. The weight room should be maintained at a constant temperature of about 20°C. Scales must be protected from exposure to heat and air currents, as well as from dampness, dust, harmful gases and shocks. To reduce the influence of air and heat flows, it is recommended to close windows and doors with thick curtains. Windows should be double glazed and sealed tightly; Windows and vents cannot be opened. It is recommended to ventilate the weighing room with a fan, and only when weighing is not in progress. It is recommended to cover the floor with linoleum, which is easy to clean from dust and is a poor heat conductor.

Scales should be installed in a horizontal position on particularly strong pedestals that protect the scales from any shocks. It is not recommended to move the scale from place to place.

Analytical balances with a maximum load of 100 g or more are recommended to be installed on a console table, consisting of a concrete slab, freely lying on shock-absorbing rubber or foam pads in the table frame, resting on two metal brackets attached to the main wall.

It is advisable to install semi-microanalytical balances on a table with massive legs. The table consists of a massive lid, into the frame of which there is felt, a reinforced concrete mosaic slab and linoleum.

The lamps in the weighing room should sufficiently illuminate the scale scale and, at the same time, not heat up the rocker arms. It is best to install fluorescent lamps.

A table with basic rules for handling scales should be posted in the weighing room.

The cleanliness of the weighing room must be carefully monitored. At the end of weighing, it is recommended to cover the scales with covers.

Do not place anything on the console table or shelf on the brackets where the scales are installed. To the left of the table (shelf) it is advisable to have a mobile table for a desiccator with a weighed substance and for making records.

Rules for using analytical balances

1. The load on the scales should not exceed the maximum for this type of scale. They weigh only while sitting against the scales, resting their hands on the table top. The object to be weighed is taken with tweezers, tongs or clean paper and placed in the middle of the left cup. Chemical substances are weighed in a glass container (bunch, ampoule). Cannot be placed chemical substances directly onto the scale or weigh on a piece of paper.

2. The item being weighed must be at the same temperature as the scale. Therefore, before weighing, the substance should be kept in a desiccator near the scales for 20-30 minutes. If, when weighing, a lamp is turned on over the scales, then this must be done 10-15 minutes before starting work.

3. The substance to be weighed should only be added or subtracted outside the scale cabinet. If the substance to be weighed is spilled on the scale pan or on the bottom of the cabinet, you must immediately sweep it up with a brush.

4. The weights should be placed on the right pan of the scale so that they are in the center of the pan. The weights should be picked up with tweezers with bone (plastic) tips.

5. When the substance to be weighed or the weights are placed on or removed from the scale, the scale must be locked.

6. Before each weighing, their zero point should be checked and, if necessary, set. While observing the deflection of the scale needle, the cabinet doors must be closed.

7. When balancing an object being weighed, start with large weights and then move on to smaller ones.

You should always use the smallest number of weights, for example, take a weight of 2 g, and not two weights of 1 g. On the scale, the weights should lie in a certain order; Small weights should not be stacked on top of each other. Large weights should be placed in the center of the cup so that it does not wobble.

Weighing errors and their elimination

Errors in accurate weighing can occur for various reasons: from the imbalance of the scales; from being suspended in air, and not in emptiness; from changes in body weight during the weighing process due to fluctuations in temperature, humidity and air pressure; from inaccurate mass values ​​of weights; from instrumental errors.

Errors from balance imbalance mostly occur with the method of simple weighing on periodic swing scales. However, corrections for unevenness are not always required. Thus, when determining the percentage composition of a substance (in % (wt.)), when weighing the analyte and its weight form is carried out on the same scales and when the substances being weighed are placed on the same cup, the relative error for both weighings will be approximately the same. But when it is necessary to determine the absolute mass of an object with an accuracy exceeding 0.1 mg, one has to resort to weighing methods that exclude corrections for unequal shoulder, for example, the substitution method.

The Borda substitution method is as follows. The mass to be measured is placed on the right pan of the scale and balanced with any tare mass on the left pan. The equilibrium position E1 is determined. Then the mass to be measured is removed from the right cup without removing the container from the left one, and instead of the removed mass, weights are placed in such an amount that it is possible to read on the scale, and the equilibrium position E2 is determined. The measurement result is equal to the mass of the applied weights plus the reading on the scale and is determined by the formula (E1 - E2)S, where S is the sensitivity of the scales.

The substitution method proposed by D.I. Mendeleev consists in placing weights on one of the cups in an amount corresponding to the maximum load of the scales, and balancing the scales with a tare weight. The body to be weighed is placed on a cup with weights and such a number of weights are removed so that the scales return to the initial equilibrium position. The value of the mass of the body being weighed is determined as the algebraic sum of the mass of the weights removed from the cup and the readings on the scale. This method is the basis for the operating principle of double-prism, single-arm scales.

Errors caused by weighing in air follow from the well-known physical law that every body immersed in a liquid (gas) loses as much weight as the liquid (gas) it displaces weighs. All bodies, therefore, weigh less in air than in empty space. Normal weighing in the air would lead to correct result, if the weights lost as much in their mass as the body being weighed loses. However, analytical weights are usually made of stainless steel (p = 8.0 g/cm3) or brass (p = 8.4 g/cm3), and milligram weights are made of aluminum (p = 2.7 g/cm3). If the density of the body being weighed is less than the density of the weights, then the body displaces more air than the weights and, therefore, it weighs less in air than in empty space. The error value usually does not exceed 0.04-0.05%.

Errors caused by changes in the mass of bodies during the weighing process can occur due to the absorption or loss of moisture, evaporation of volatile substances, temperature changes, inattention and carelessness of the experimenter. These errors can be eliminated by weighing the substances by difference in a hermetically sealed small glass container. When weighing by difference, the position of the zero point can be ignored.

Errors in the mass of weights depend on the degree of accuracy in adjusting their mass to the nominal value, the certification error and on irreversible changes in mass during the testing period, mainly due to corrosion. Errors associated with the inaccuracy of the masses of the weights used can be eliminated by comparing them with the mass of standard weights on the scales on which they will be used.

Micro and ultra micro balances

For particularly accurate measurements of small masses when conducting physicochemical studies and microanalyses, precise lever and leverless scales of various designs are used.

Lever spring scales are produced with a maximum load from 20 to 100 mg, with a division price of 10 minus 7 – 10 minus 5 mg (VLU-20 mg and VLU-100 mg). The principle of operation of these scales is based on balancing the moment created by the mass being measured by twisting a quartz stretcher. By design, these are scales on braces with an equal-arm beam and a zero weighing method. The rocker arm is placed in a special container that protects it from the action of air currents and at the same time serves as a heat distributor. The cup with the load to be weighed is carried out from the rocker column into the side compartment of the display case by a manipulator, which is interlocked with the mechanism for opening and closing the columns. The measurement results are counted on the scale of the measuring dial, the division value of which is 0.00032 mg (VLU-20mg) and 0.0005 mg (VLU-100mg). The time to calm down the vibrations of the rocker arm is about 1.5 minutes.

Microanalytical balances VLM-1g are intended for weighing precious stones and metals, as well as various substances for microchemical analyzes of increased accuracy. The scales have equal-arm rockers with two pendants and cups. Full mechanical weight bearing is carried out by two kettlebell mechanisms. The scales are equipped with a mechanism for removing the left cup. Measurement range on optical scale ±1 mg. The optical scale division value is 0.01 mg. Weighing error ±0.07 mg.

To quickly determine the mass of very small quantities of substances, torsion (spring) balances are often used (Fig. 80). They differ from quadrant ones in that the load-receiving cup in them is enclosed in a display case and equipped with a locking device. Torsion bar scales are available with different weighing limits. In laboratory practice, the VT-500 model is often used. The maximum permissible load of the scale is 500 mg, and the smallest is 10 mg. The absolute error of readings at any scale mark is no more than ± 1 mg.

The measuring element in a torsion balance is a spring, the tension of which, when twisted, balances the sample being weighed. The angle of twist of the spring is proportional to the mass of the sample being weighed, therefore the scale scale is graduated in mass units.

When using torsion bar scales VT-500, they are installed at level 1 using support screws 2, after which the rocker arm 3 is released by moving the securing lever 4 to the right. The mass indicator 5 is set to zero using the tension lever 6. With this position of the scales, the balance indicator 7 overlaps the balance line or it is brought to this position by the calibration head located on the back of the scale in the center. Then secure the rocker by moving the securing lever to the left as far as it will go, and begin weighing. To do this, open the safety cover 8, hang the load to be weighed on the hook of the rocker arm 9 and close the cover again. The rocker arm is released by moving lever 4 to the right. By turning lever 6 to the left, move pointer 5 until pointer 7 is positioned exactly on the balance line. In this position, pointer 5 shows on the scale the mass of the load being measured. After weighing, secure the rocker by moving lever 4 to the left, open cover 8, remove the weight from the hook and close the cover. Lever 6 is moved to the right, pointer 5 is set to zero - and the scales are ready for the next weighing.

The purpose of the work is to learn how to use lever scales and determine the mass of bodies with their help.

Equipment and materials: scales, weights, several small bodies different weights, jar, shot or dry clean sand.

Directions for use

1. Read the appendix to the work “Weighing Rules”.
2. Following the rules of weighing, measure the mass of several solids accurate to 0.1 g.

Additional task

There is a special weighing method called the taring method. When weighing using this method, an object whose mass they want to determine is placed on the left pan of the scale. A jar is placed on the right cup, in which pour dry sand or fine shot until until the scales come into balance. Then the object is removed from the left pan of the scale and weights are placed in its place and with their help the scales are brought into balance. The mass of these weights will be equal to the mass of the object. The taring method can be used to measure body weight quite accurately even on slightly out-of-tune scales.

Test this out. Place a pellet or a wad of paper on the left pan of the scale; this will upset the balance of the scale - the scale will go out of balance. Measure the masses of the bodies you have using the taring method and compare them with the result obtained when weighing on balanced scales.

Think and explain why the taring method can be used to measure body weight quite accurately even on slightly out-of-tune scales.

Weighing Rules

1. Before weighing, you must ensure that the scale is properly balanced. If necessary, to establish balance, you need to place strips of paper, cardboard, etc. on a lighter cup.
2. The body to be weighed is placed on the left pan of the scales, and the weights are placed on the right.
3. To avoid damage to the scales The body to be weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.
4. You cannot weigh bodies heavier than the maximum load indicated on the scale.
5. Do not place wet, dirty, or hot bodies on the scales, pour powders without using a liner, or pour liquids.
6. Small weights should only be picked up with tweezers (Fig. 310).

Having placed the body to be weighed on the left pan, a weight having a mass slightly greater than the mass of the weighed body is placed on the right one ( selected by eye followed by inspection). If this rule is not followed, it often happens that there are not enough small weights and you have to start weighing all over again.

If the weight overtightens the cup, then it is put back in the case, but if it doesn’t overtighten, it is left on the cup. Then the same is done with the next most important weight and so on until there is equilibrium has been achieved.

Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred from the scale pan to the case.

Laboratory work No. 3 page 161

Goal of the work: learn to use lever scales and use them to determine the mass of bodies.

Devices and materials: scales with weights, several small bodies of different masses.

Weighing – a method of measuring mass using scales.

Other units of mass:

1 t = 1000 kg

1 c = 100 kg

1 g = 0.001 kg

1 mg = 0.000 001 kg

Safety regulations.

1.Be careful with scales. Follow the weighing rules.

2. There should be no foreign objects on the table.

3. Place the scale in the middle of the table.

4. Do not lose weights and weights, especially do not put them in your mouth!!!

I have read the rules. I undertake to fulfill . ______________________

/Student's signature/

Weighing rules.

• Before weighing, make sure that the scales are balanced. If necessary, to establish balance, you need to place strips of paper, cardboard, etc. on the lighter pan of the scale.
• The body to be weighed is placed on the left pan of the scales, and the weights are placed on the right

Weighing rules.

3. To avoid damage to the scales, the body being weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.

4. You cannot weigh bodies heavier than the maximum load indicated on the scales. (200g.)

Weighing rules.

5. Do not place wet, dirty, hot bodies on the scales, pour powders without using a liner, or pour liquids.

6. Small weights should only be picked up with tweezers.

Weighing rules.

7. Having placed the body to be weighed on the left pan, a weight having a mass slightly greater than the mass of the body being weighed is placed on the right pan (selected by eye and then checked). If this rule is not followed, it often happens that there are not enough small weights and you have to start weighing all over again. If the weight pulls over the cup, then it is put back into the case, but if it doesn’t, it is left on the cup. Then the same is done with the next weight of smaller mass, etc., until equilibrium is achieved.

Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred from the scale pan to the case.

Check that all the weights are placed in the case and that each of them is in its intended place.

Training tasks and questions

• What physical quantity is determined using lever scales? ____________________

2. In what units is it measured (name all)?

________________________________

3.Do the exercises:

8.4 t = _______ kg

0.5 t =________ kg

125 t =________ kg

500 mg = ________ g

120 mg =_________ g

60 mg = _________ g

4.100 g +20 g + 2 g + 1 g +500 mg + 200 mg =___g

20 g + 10 g + 1 g +200 mg + 100 mg =_________g

5.Which cup is it placed on?

body being weighed? on ____________

weights? on ________________

6.What needs to be done on a lever scale before weighing?_____________

Progress.

1. Following the rules of weighing, measure the mass of several solids to the nearest 0.1 g.

Progress.

2. Record the measurement results in the table.

№ experience

Body name

Body mass

m , G

Cube

Body mass

m , kg

Body weight on electric scales m , G

Conclusion:

I learned to use lever scales and with their help measure the mass of various bodies with an accuracy of .......

Additional task.

• Which weights from the school set must be placed on a cup of educational scales in order to balance a piece of sugar weighing 10.50 g lying on another cup? (set of weights: 10g, 5g, 5g, 20mg, 20mg, 10mg).
• Express the mass of bodies in kilograms: 3.5t; 0.25t; 150g; 15
• How many grams are in 7.5 kg?
• The mass is designated by the letter ……….
• 100g + 20g + 2g + 1g + 500mg + 200mg =…..

Homework

§19, 20

Exercise No. 6 (1, 2, 3)

Goal of the work:

Devices and materials:

WEIGHING RULES

In what units is it measured (list all)?

_____________________________________________________________

Do the exercises:

8.4 t = ___________ kg 500 mg =____________ g

0.5 t = ___________ kg 120 mg = ____________ g

125 g= ___________ kg 60 mg = _____________ g

100 g+ 20 g + 1 g 500 mg + 200 mg = ___________________________ g

20 g+ 10 g +1 g + 200 mg + 100 mg = ___________________________ g

Which scale pan is placed on:

body being weighed?____________________

weights?___________________________

PROGRESS

№ experience

Body name

Kettlebells

Body weight, g

Conclusion:____________________________________________________________

Laboratory work No. 3 “Measuring body weight on lever scales.”

Goal of the work: learn to use lever scales and use them to determine the mass of bodies.

Devices and materials: scales, weights, several small bodies of different masses.

WEIGHING RULES

Before weighing, make sure that the scales are balanced. If necessary, strips of paper should be placed on a lighter cup to establish balance.

The body to be weighed is placed on the left pan of the scale, and the weights are placed on the right.

To avoid damage to the scales, the body being weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.

You cannot weigh bodies heavier than the maximum load indicated on the scale.

Do not place wet, dirty, or hot bodies on the scales, pour liquids, or pour powders without using a pad.

Small weights and weights should be picked up with tweezers.

Having placed the body to be weighed on the left cup, a weight having a mass close to the body weight (by eye) is placed on the right one.

If the weight pulls over the cup, then it is put back in the case; if not, it is left on the cup. Then weights of smaller mass are selected in the same way until equilibrium is achieved.

Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred to the case.

TRAINING TASKS AND QUESTIONS

What physical quantity is determined using lever scales? weight

In what units is it measured (list all)? In SI - kg, in l/r - g

Do the exercises:

8.4 t = 8400 kg 500 mg =0.5 g

0.5 t = 500 kg 120 mg = 0.12 g

125 g= 0.125 kg 60 mg = 0.06 g

100 g+ 20 g + 1 g 500 mg + 200 mg = 121.7 g

20 g+ 10 g +1 g + 200 mg + 100 mg = 31.3 g

Which scale pan is placed on:

body being weighed? left

weights? right

What needs to be done on a lever scale before weighing?

Before weighing, make sure that the scales are balanced. If necessary, strips of paper should be placed on a lighter cup to establish balance.

PROGRESS

Knowing the rules of weighing, measure the mass of several small bodies with an accuracy of 0.1 g.

Record the measurement results in the table:

№ experience

Body name

Kettlebells, with which the body was balanced

Body weight, g

Conclusion: the mass of the body is approximately equal to the sum of the masses of the weights balancing the scales.